1. Field of the Invention
The invention relates to an electric motor vehicle. The “electric motor vehicle” in this specification includes a hybrid vehicle equipped with both an electric motor and a combustion engine, and a fuel cell vehicle.
2. Description of Related Art
An electric motor vehicle is equipped with an inverter that converts direct-current power of a battery into alternating-current power of a frequency that is suitable to drive an electric motor. Many an electric motor vehicle is designed so that at the time of braking, inertia energy of the vehicle is utilized to cause an electric motor to generate electric power, and the generated electric power is then converted by an inverter into direct-current power, with which the battery is charged.
For example, in the case where a three-phase alternating-current electric motor is employed, the inverter outputs alternating electric currents of three different phases. In order to produce three-phase alternating current from direct-current power, the inverter has, as a main circuit, a construction that includes three parallel connected circuit sets in each of which two switching elements are interconnected in series. An output line of the electric motor extends from an intermediate point of the series connection between the two switching elements of each circuit set. The two series-connected switching elements are referred to as “arm”. Furthermore, of the two series-connected switching elements of each circuit set, the current path that extends through the higher-voltage switching element is often referred to as “upper arm”, and the current path that extends through the lower-voltage switching element is often referred to as “lower arm”.
In some types of vehicles, the inverter and the electric motor are always electrically connected together. In an electric motor vehicle of such a type, if one of the switching elements has short-circuit failure and, at the same time, the electric motor is generating electricity, excessively large current may flow through the short-circuited switching element or a different specific site, so that the circuit may be damaged. Alternatively, excessively large current may reversely flow into the electric motor, so that a motor cable may be damaged or the permanent magnets of the electric motor may lose magnetism.
Examples of technologies for avoiding such damage are disclosed in Japanese Patent Application Publication No. 2007-306720 (JP 2007-306720 A), Japanese Patent Application Publication No. 2008-182842 (JP 2008-182842 A), and Japanese Patent Application Publication No. 2010-068689 (JP 2010-068689 A). In the technologies of JP 2007-306720 A and JP 2008-182842 A, if a switching element of the inverter has short-circuit failure, the inverter and the electric motor are disconnected to each other. In the technology of JP 2010-068689 A, if the switching element of an upper arm in the inverter has short-circuit failure, a controller causes the switching elements of the other upper arms to be short-circuited. Also, if the switching element of a lower arm in the inverter has short-circuit failure, the controller causes the switching elements of the other lower arms to be short-circuited. For this operation the current generated by the electric motor is dispersed.
To employ the technology of JP 2010-068689 A, electric power for controlling the switching elements is required. Usually, the controller of the inverters receives electric power for control from a battery. Therefore, as long as the electric power is supplied from the battery to the controller, the technology works satisfactorily. However, the electric power is not supplied to the controller, for example when the battery fails, or when a cable between the battery and the inverter is broken. Furthermore, if a vehicle is towed for a long time, the battery may become low in voltage and unable to supply sufficient electric power to the controller. A specific electric motor vehicle may have a high-voltage main battery that supplies electric power to the electric motor and an accessory battery. The accessory battery supplies electric power to low-voltage devices termed accessories, such as the control circuit of the inverter, a room lamp, etc. The electric power is supplied to the accessory battery from the main battery with reducing the voltage. In such a type of electric motor vehicle, when the main switch of the vehicle is off, the system main relay is also off; therefore, a voltage step-down converter for reducing the voltage supplied from the main battery to the controller can be stopped. Hence, during tow of the vehicle when the main switch of the vehicle is off, the accessory battery is the only battery that supplies electric power to the controller. If the vehicle is towed for a long time, the accessory battery may become low in electric power so that the inverter cannot be operated by the controller.